MAINT: Various minor code cleanups.

Minor cleanups of old code to reflect more modern usage.

1 files changed, 2 insertions, 2 deletions

diff --git a/numpy/lib/_iotools.py b/numpy/lib/_iotools.py
index b7db77f32..27143e5c6 100644
--- a/numpy/lib/_iotools.py
+++ b/numpy/lib/_iotools.py
@@ -861,8 +861,8 @@ class StringConverter(object):
         # Add the missing values to the existing set or clear it.
         if missing_values is None:
             # Clear all missing values even though the ctor initializes it to
-            # {''} when the argument is None.
-            self.missing_values = {}
+            # set(['']) when the argument is None.
+            self.missing_values = set()
         else:
             if not np.iterable(missing_values):
                 missing_values = [missing_values]